Did you take one of the Irreligion civics, Secularism or State Atheism? If so that is the cause. There was a religious grievance which went away that cause the error. If that's the case load your oldest autosave and check the war resolution/ grievance screen and see if its empty like that. If it is empty sorry your out of luck, if it looks normal don't take the civic.

Here is a post about it
and this one is mine

Yes, I picked secularism couple of turns back and then I canceled it. Unfortunately I don't have the savegame before that as they are all overwritten. But I loaded an autosave for 6-7 turns back, when I still didn't choose Irreligion civics and continued it without choosing any of them and it worked! I was able to make a force surrender without problem. 

Hi, I have this same bug where I cannot accept terms of surrender of my enemy and also cannot advance the turn.  I checked the Irreligion civic as suggested.  I had unlocked this civic choice quite a while back, but not selected an option yet.  Selecting an option doesn't seem to fix the bug.  Could it be that the enemy picked one of these civics during the war and that caused the bug?  Please help, this is my first game and close to the end, but now I'm stuck!